Innermost part (4)
I believe the answer is:
core
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">innermost part</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for core that I've seen before include "Essence" , "essential element" , "Central part of an apple" , "Centre, heart" , "Nucleus" .)